1. Field of the Invention
The present invention relates to a carbon fiber having a novel cross-sectional structure and improved strength.
2. Discussion of Background
Carbon fibers have high specific strength and high specific modulus, and they are expected to be most prospective as filler fibers for high performance composite materials. Among them, pitch-type carbon fibers have various advantages over polyacrylonitrile-type carbon fibers in that the raw material is abundantly available, the yield in the carbonization step is high, and the elastic modulus of fibers is high.
Various studies have been made for the preparation of pitch material having good orientation properties for spinning, since it has been reported that it is possible to obtain pitch-type carbon fibers having high quality by using a pitch wherein carbonaceous raw material is heat-treated to develop anisotropy and readily orientable molecular seeds are formed, instead of an isotropic pitch which has been commonly used as the pitch material for spinning (Japanese Examined Patent Publication No. 8634/1974).
It is well known that when a carbonaceous raw material such as heavy oil, tar or pitch is heated at a temperature of from 350.degree. to 500.degree. C., there form, in the material, small spherical particles which have a particle size of from a few microns to a few hundred microns and which exhibit an optical anisotropy under polarized light. When further heated, these small spherical particles grow and are integrated to form a structure having an optical anisotropy. This anisotropic structure is considered to be a precursor for a graphite crystal structure, wherein planar polymeric aromatic hydrocarbon layers formed by the thermal polycondensation of the carbonaceous raw material are laminated and oriented.
A heat-treated product including such an anisotropic structure is generally called mesophase pitch.
As a method for using such mesophase pitch as the pitch material for spinning, there has been proposed a method wherein e.g. petroleum pitch is subjected to heat treatment at a temperature of from about 350.degree. to about 450.degree. C. under a stand-still condition to obtain a pitch containing from 40 to 90% by weight of a mesophase, which is used as the pitch material for spinning (Japanese Unexamined Patent Publication No. 19127/1974).
However, it takes a long period of time to convert an isotropic carbonaceous raw material to the mesophase pitch by such a method. Under the circumstances, there has been proposed a method wherein the carbonaceous raw material is preliminarily treated with a sufficient amount of a solvent to obtain an insoluble component, which is then subjected to heat treatment at a temperature of from 230.degree. to 400.degree. C. for a short period of time, i.e. for 10 minutes or less, to form a so-called neomesophase pitch which is highly oriented and contains at least 75% by weight of the optical anisotropic component and at most 25% by weight of quinoline-insoluble components, and the neomesophase pitch is used as the pitch material for spinning (Japanese Unexamined Patent Publication No. 160427/1979).
As other pitch materials having good orientation properties for the production of high performance carbon fibers, there have been proposed a so-called pre-mesophase pitch, i.e. a pitch which is obtainable by subjecting e.g. coal tar pitch to hydrogenation treatment in the presence of tetrahydroquinoline, followed by heat treatment at a temperature of about 450.degree. C. for a short period of time and which is optically isotropic and capable of being changed to have an optical anisotropy when heated at a temperature of at least 600.degree. C. (Japanese Unexamined Patent Publication No. 18421/1983), or a so-called dormant mesophase, i.e. a pitch which is obtainable by subjecting a mesophase pitch to hydrogenation treatment e.g. by the Birch reduction method and which is optically isotropic and, when an external force is applied, exhibits an orientation to the direction of the external force (Japanese Unexamined Patent Publication No. 100186/1982).
It is possible to obtain pitch fibers by melt spinning such pitch material having good orientation properties through spinning nozzles. Then, the pitch fibers may be subjected to infusible treatment and carbonization, and optionally to graphitization, to obtain pitch-type high performance carbon fibers
When the above-mentioned pitch material having good orientation properties is melt-spun, the laminar structure of planar polymeric hydrocarbon in the resulting pitch fibers is likely to have radial orientation in the cross-section of each fiber. Carbon fibers are commonly used for various fiber-rainforced composites, of which matrixes are made of e.g., an epoxy resin, a phonol resin or aluminum. In such cases, not only the strength of carbon fibers but also the bonding properties of carbon fibers with the matrix are important. As mentioned above, the carbon fibers having radial orientation in their cross-section generally have good bonding properties with the matrix, and they are preferable in such as aspect. In such pitch fibers, however, there have been drawbacks such that when tensile stress is exerted in the circumferential direction of the cross-section of each fiber due to the carbonization shrinkage during the subsequent infusible treatment and carbonization treatment, wedge-shaped cracks extending in the axial direction of each fiber are likely to form in the cross-section of the resulting carbon fiber, whereby the strength of the fiber tends to deteriorate. In an extreme case, the commercial value of the carbon fibers is impaired.
Conventional commercially available pitch-type carbon fibers have radial orientation or random orientation in their cross-sectional structure. Thus, they are weak against compression in a radial direction, although they are strong in a longitudinal direction. Accordingly, when they are used for a composite, the mechanical strength of the composite tends to be poor. As a reason for this, it is believed that since stractural units in the cross-section of such commercially available carbon fibers are coarce, they are likely to cleave along such structures when a radial force is exerted to them.